Measuring and monitoring circuits for telephone lines



F. A. cowAN MEASURINGAND lMONITORING CIRCUITS 'FOR TELEPHONE LINES yFiled sept 25. 1929 INVENTORY.

ATTORN EY on other measuring apparatus at other pointsv on the line. The purpose of this invention 1s Patented Feb. 24, Y19371 Umran sraraa meats-s tartar l ori?icl;-v

' FRANK A. cowA'N, or EAST ORANGE, NEW JERSEY, AssIGNoR TofAivIERIoAN TELE-l PHONE ANI) 'IELEGRAPH COMPANY, AKCORPORATION 0F NEW YORK MnasURINe AND MONITORING CIRCUITS FOR Tn'LnrIIoNn 'LINES Application inea septemter'a5`,r1929 serial Noyaeaclie.

This invention relates to telephone circuits, and in particular to means for monitoring, 'or measuring the transmission characteristics thereof.- Y w The usual 'practice in taking measurements on a telephone line is to impress. an alternating current of known frequencyv and amplitude on the line at one point and to take measurements on the amplitude or energy level at other points, thus giving information as to the losses or gains due to the line or l apparatus introduced into the line.

In general, Awhile'such Vmeasuring or monitoring Vapparatus is bridged across the line and is of high impedance, it may react on the line in such a manner as to modify its charac teristics or may alter the readings to be taken to'avoid sucheffects by apcircuit arrangement in which the reaction on the line is zeroor is independent of the changes in the measuring apparatus, and also in which the measurements at any Vpoint are independent of changes made on. the line beyond the point of measurement. A y.

To this end I provide a circuit in'which the measuring apparatus is connected in conjugate relationship to the portion of the transmission line beyond the measuring point.,

. The invention will be better understoodby reference to the following specification, taken r in connection with the accompanying drawing, in which Figure lv is a V:diagrammatic drawing to assist in the vexplanation of the following iigures; Fig. 2 is a circuit showing the application in one of its simpler forms in connection with a repeater; Fig. 3is a'modication of Fig. 2 arranged to give symmetry to the two sides of the transmission line; Figs. 4 and 5 are analogous to Figs. 2 and 3, respectively, and show the invention applied to a line independently of any repeaters or similar apparatus which maybe associated there'-4 to; andFig. 6 shows. a method -by which the change in energy level due to a piece'of apy lparatus, such as an amplier, may be made ,Y

:without disconnecting it from the transmission line.

Y,The operation of the invention depends es;

sentiallyupon a well known property of the l when the tour impedances have theY relation whatever happens in one diagonal is entirely Fig. lofY the drawing f independent of what takes place in the other f diagonal.

For example, if a source of v F. acts in thearm fa., itlwillproduce current,

sees in both of the diagonals, but any change in f one diagonal will have noeffect on whatftakes Y place in the other. It one ofthe diagonals contains a source of E. M. F., nocurrentdue to this source will flow in the other diagonal:`

fro'

Under these conditions Vthe, YVheatstone bridge is'said to be balanced, and the two'diagonals constitute two conjugate branches. l

f In Fig. 2 there is shown an incoming telephone line l: vand an outgoing line O, with a vacuum tube ampliiierkr'or repeater A intro# duced. Across the `input of the` amplifier there is bridgedan impedance Z1 of a suitable value to match in a desired wayl with the incoming transmission line. The outgoing line is shown here as associated with the-amplifierA by means of a transformer T, this beingof a suitable turn ratio to match the impedance of Y the repeaterset to the impedance of the outgoing line. .Theresistance RS, R1 and R2 are connected in series with the platelcircuit of the vacuum tube, and the primary of theoutput transformer is connected in parallel with the resistances R1 and R2. These three vresistances, together with the A. C. plate circuit resistance R0 oi' the vacuum. tube, form a Wheatstone bridge of which the output transformer forms one diagonal Vand the connection' to "the measuring apparatus lforms. 'the other.

If these resistances are so chosen that i vRo i "R2 the `bridge is balanced, the-'voltage in pressed on the level measuring apparatus by waves amplified in the vacuum tube is independent of the impedance connected to the output transformer or to changes in the said impedance. If EO is the E. M. F. acting in the platecircuitot thev vacuum tube, then El, the voltage acting on the level measuring apparatus, assumed to be of high impedance,

l1s given by Also, if the repeater impedance has been matched with the outgoingV line' in accord.- ance with well established practice, then EL, the voltage impressed upon the outgoing` line, neglecting losses due to resistances and transformer, is given approximately by Vwhere RL is the impedance of theline for which the output' transformer is designed; that is, the impedance of the outgoing line as seen from the secondary of the transformer. The level that'would be impressed on the nominal line would, of course, be greater than that indicated bythe level measuring device byfthe following factor: t Y

i, i EL 4 1 Ro Rn KI 2() log 10E-20 log l0 l -l-FS R-O Having found this factor once for all for a particular circuit, the. proper value of theV transmission level asimpressed on an outgoing'f'line having normal impedance can be calculated from observations made on the measuring apparatus. Y

The measuring apparatus may take on a Avariety of forms, and in Fig. 2 ithas been shown in simplified manner'as consisting of a detector and a' meter M of any appropriate character: It is also seen that'` theresistances'Rg, R1 and R2 wou'ld'always be at the potential of the plate battery withl respect to ground. rlhis might' cause undesired deflections of the'meter" ofthe measuring apparaf tus, but such effects can'be'ravoided: by convnectin'g'the condensers C1 and C2 in series' sliouldbe made as large as possible inorderk to avoid-excessive loss on vthrough transmis-1 sion, but should notf be large enough to cause" appreciable error in the indication'of the" transmission measuring apparatus, this latter being preferably,r of still higher impedance. Also, RS should betalien as low as feasible in order that the transmission loss due toit shall be' keptj reasonably small'. For example, RL;

tubes.

'the' vacuum tube.

might be of the order of several thousands of ohms, and RS might very well be of the order of some tens of. ohms.

It is to be noted that if the observer at the level measuring device at an intermediate point on a long repeatered circuit should un intentionally operate the key which sends out testing current, this current would not reach the line in either direction because the vacuum tube wouldblock transmission towards the transmitting end of the circuit and the lheatstone bridge arrangement of the resistances would prevent transmission towards tlieieceiving end ofthe line, this resulting from ,the conjugate relationship between the measuring circuit `and the outgoing line. Notonly does this referto the closing of the 'key lof the testing circuit but would relate to any other changes made in the measuring circuit.

It will also be noted that in Fig. 2 the leads to the measuring apparatus are not balanced with respect to ground, since the one containing; Ci isv effectively connected t-o A. C.

groundpotential'while the one containing?" the condenser@2 has potential with respect to ground,which'might be a cause of crosstalk between repeater circuits; Fig. 3 shows the application 'of this. invention to a pushl pull 'amplifier circuit in which symmetrycanv be obtained-so as to avoid Athe'lack of balance just noted, as well'y as certain'other advantages. In this Fig. 3 two sets of resistances are4 provided, one foreach lof the vacuum The value of these Vresistances arejt determined as in Fig. `2. R., is -now taken as representing the A. C. impedance of the plate circuits of .the twoY vacuum tubes' in series. The two parts of the high side of the` output transformer are connected respectively",`

yaround the corresponding resi'stances R1` and R2, and the resistances *Risarey connected between the two parts of the primary ofthe transformer. .Thus,' it' is seen that the ar-M rangementv for 'the two tubesitalren together is essentially the same as that of Fig. 2, and the method of "calculating the' `values of these resistancesand the losses caused by-t'hem isV thesame as for Fig. 2Qv It will be noted, liowjv x ever, that the leadsto the measuring apparai" tus are balanced with respect to ground and therefore are. less likely to pick up crosstalk than is the case with the unbalanced arrangement ofFig.l 2. The sameconnections ofA condensers Ci and C5A andv leaks which' are` described in connection with Fig. 2 should also be usedinthecircuit of Fig. 3 in order te. keep they platev battery potential away -from the measuringapparati'is.

Fig. @shows aY circuit for applyingthel principles of this AinventionV to'measu'rements at'some otherp'ointthan theplate circuit of In thiscase R0 is the im.- pedancer` looking'towards the source from which thewaves arej coming; In' order to avoid unbalance a resistance. RS', which is in the impedance of the line which comprises a `Wheatstone network, the incoming line con- Y equal to RS, should ,beY connected in the oppo site Side ofthe'c'ircuit, and R., should be con- Y stituting one side thereof, the outgoing line Sidered "as" including this additional vresistconstituting one diagonal thereof, Yand the measuring apparatus constituting the other *"ln this circuit, as in Fig. Q, there is a lack diagonal. l Y

of balance whic might lead to crosstalk be- 2. ln a transmission line, means for meas-A transmission level without aecttween different circuits, but if a transformer is available, such as in Figs. 2 and 3, then this laclr of balance can be avoided by the connection of Fig. 5, which, itwill be recognized, inV is essentially similar to Fig. 3.

, f Fig. 6 shows an arrangement by which the ain of an amplifier, or the gain or'loss due to any other piece of apparatus, may be measured without removing. it from the circuit. ln this case v and a pairo leads to the ridged across the input terminals of the amplifier at the same point. -K By adjusting the output wave of sutable volumeV or their equivalents. VThe measurements thus give the gain due to the amplifier as working into the actual circuit with which it is associated. v

lt has been assumed heretofore that the impedance of the measuringcircuit'is very high but the invention'is notlimitedto this condition, for if the measuring-apparatus is one whose impedance is not high there will be merely a correction factor lof constant value to be determinedeither by a separate measurement or by simple calculations appropriate to such circuits.

Also, it will be noted that while the inven-V tion has been described specifically in con-A nection with the measuring of energy'leve it is equally well adapted as a means for monitoring a circuit which is free from the possibility of disturbing transmission on the` circuit. l

A further use of these arrangements is to rovide a directionally selective bridge connection for a transmission line, this resulting from the conjugate relationship previously mentione If, for example, resistances are connected as shown in Figs.. i and at an intermediate point on a two-way transmis-l sion line, apparatus connected to the terminals heretofore descriY bed as those for the measuring apparatus can receive waves from or transmit waves to the line extending towards the incoming line, but can not receive waves from or transmit waves to the line in the opposite direction.

uring the ing the impedance of the'y line, comprising series and bridge resistances; the

coming line, the series resistance an bridge vresistance constitu going line and the measuring stituting respectively the two said bridge. Y c v v 3. ln a vacuum tube telephone-repeater, an outgoing line and a measuring circuit in the output of said repeater and iugate relationship. Ll. ln a transmission name to September,

said means t e the four arms a balanced Wheatstone bridge, the out-4 apparatus condiagonals of ting arranged in conline, a push-pull A.re-

measuring apparatus is connected.

Y In testimony whereof, this specification have signed this 944thV day of of the oscillator a may be'impressed upon the input of the ampeater, means for measuring energy level at plifler, and the level of the Awave at this point ya point on the line, said means comprising may be measured by the level measuring eresistances in the youtput of the repeater and vice. The level of the output terminals o so associate therewith as to yieldl a` symthe amplifier may be measure 'as described metrical circuit with two pairs of conjugate above, it being understood that the amplifier-"points, to one of which the outgoing line is circuit includes the resistances B1,R2 and s, connected and to the other of w ic the i929. Y Y `FRANK A. Cowan; y

ilo

line,

means for measl without a 

